1. Field of the Invention
The invention relates to a camshaft and, more particularly, relates to a method of producing a cast iron camshaft having a hardened layer formed by a high-density energy source such as a tungsten inert gas (TIG) arc, a laser beam, and an electron beam.
2. Description of the Prior Art
Three methods have been proposed for hardening a sliding circumferential surface of a cam of a cast iron camshaft.
1 By adopting an induction hardening treatment.
Generally the entire sliding surface of a cam is hardened by the induction hardening method to form a surface hardened layer having a martensite structure. The hardened layer has a good wear resistance to fatigue spalling (flaking), i.e., so-called resistance to pitting, but has a low durability in terms of its resistance to scratch wear, i.e., so-called scuffing resistance. To ensure a high hardness of the hardened layer by a high frequency induction hardening method, a larger amount of hardenability improving elements, such as Cr, Mo, and Ni, should be added to the cast iron material. Such addition increases the percentage of defective castings, and thus the rejection rate, and inevitably the costs, are increased. In this case, the cast iron camshaft as cast has a perlite matrix structure including free cementite and, unavoidably, some free ferrite. When the camshaft having the above structure is hardened by the high frequency induction hardening method, the prescribed hardness of the hardened layer can not always be attained. Also, since the camshaft includes free cementite, it is difficult to properly machine the casting because of its poor cuttability. For example, great care is needed when drilling an oil hole in the cam. Furthermore, in the high frequency induction hardening method, cracks occur frequently during the quenching process.
2. By a casting method using a chill.
In this case, a mould is provided with a chill defining a circumferential surface of a cam on a camshaft. Molten iron is cast in the mould to produce a camshaft having a cam with a chill-hardened surface layer. This method is disadvantageous in that control of the hardening depth is difficult, drilling the oil holes and a main oil bore becomes difficult, and costs are increased due to the necessary production and maintenance of the chill, and to the setting and removal of the chill. Moreover, since the chill-hardened layer has a rough structure and a large amount of free graphite, satisfactory durability in sliding surface properties, such as scuffing resistance and pitting resistance, cannot be obtained. Therefore, design changes become necessary for decreasing the pressure per sliding area to ensure good lubrication and the like.
3. By melting a sliding surface of a cam of an iron camshaft by applying a high-density energy source to and quenching that surface.
As mentioned previously, this high-density energy source should be a TIG arc, a laser beam, or an electron beam. The hardened layer obtained thereby has better sliding properties, compared with layers produced by the first and second hardening methods. This third hardening method has been proposed in, for example, Japanese Examined Patent Publication (Kokoku) No. 57-6494 (publication date: Feb. 5, 1982, priority: DE Patent Application No. 2741567.4), and Japanese Unexamined Patent Publication (Kokai) No. 59-23156 (publication date: Feb. 6, 1984). In this case, the cam should be preheated to 250.degree. C. to 400.degree. C. prior to irradiation by the high-density energy source, and therefore a suitable heating device, e.g., a heating furnace, should be provided.